A Chinese study team led by Prof. Alexandre Gontcharov, a "Thousand Talent Program" member at Institute of Solid State Physics, Hefei Institutes of Physical Science, presented a new way to synthesize the pure high temperature superconducting phase of H₃S by chemically reacting molecular hydrogen and sulfur at high pressures.

Superconductivity at temperatures close to 200 K in the H-S system at high pressures attracted a great deal of excitement in recent two years. The key question is clearly about the mechanism of superconductivity, which is still a challenge especially to experimental studies because the relevant researches at such extreme environments remain scarce.

"Most of the theoretical studies consider Im-3m H₃S as the superconducting phase near 150 GPa. However, there are still other reports that challenge the decomposition scenario of H₂S or suggest a compound with a modified crystal chemistry SH-H₃S", Alex explained, "These controversies will never end unless you can synthesize a pure superconducting phase with a novel method, show them what the material is".

In fact, a series of experiments were performed in the laser-heated diamond anvil cell with 50–300 μm central culets up to a desired pressure (approximately 50 and 150 GPa). Synchrotron x-ray diffraction and Raman spectroscopy were both used to trace the products in the experiment.

In case of the multiphase in nature following the recipes of previous studies, here they developed a new synthesis way with elemental reagents, sulfur and molecular hydrogen, which led to perfectly crystallized Im-3m H₃S in the chamber.

The study team found that first-principles theoretical studies of structure and composition correctly predict the Im-3m(R3m) H₃S compound as the most stable at 110–150 GPa, and this phase was (meta)stable down to at least 70 GPa , which resolved the controversy about the superconducting phase in the H-S system.

The result of this research was published in Physics Review B in an article entitled Stable high-pressure phases in the H-S system determined by chemically reacting hydrogen and sulfur.

Figure.1 XRD patterns of Im-3m H₃S directly synthesized from H₂ and S with Comparison of the H₂S precursor. (Image by JANG Shuqing)